The aim of study was to evaluate foot and ankle muscle strength in patients with RA and age and sex-matched controls. It was hypothesised that the foot and ankle muscle strength would be reduced in those with RA. This study has shown a significant reduction in plantarflexion, eversion and inversion muscle strength in patients with RA. It is not clear from our design whether foot and ankle muscle reduction precedes development of RA or is a consequence of the disease.
The gastrocnemius-soleus muscle group are strong plantarflexors of the ankle, with the tibialis posterior muscle being the most powerful invertor of the rearfoot and a contributor to ankle plantarflexion [10, 11]. While speculative, the reduction of the gastrocnemius-soleus muscle group strength may lead to altered gait strategy. Previous studies have suggested that the weakening of the calf muscles is mainly a result of pain-avoiding strategies through altered patterns of muscular activity in RA patients [12–14]. Although the current study did not undertake 3D kinematic or kinetic analysis of the lower limb and ankle, a recent study observed abnormal tibialis posterior EMG activity in a cohort of patients with RA, and ultrasound confirmed tibialis posterior tenosynovitis in the presence of suboptimal biomechanics and moderate levels of tendon disease .
The strength ratios between muscle groups in RA have been reported with the quadriceps/hamstring strength ratio being the most commonly investigated . Again, only limited information is available about the relationship between plantarflexion/dorsiflexion and inversion/eversion strength ratios in RA. We found plantarflexion/dorsiflexion being stronger in the control group compared to RA cases. We also found that the inversion/eversion ratios were essentially the same for the RA cases and for the control group. RA has been reported to be associated with metabolic changes leading to a loss of muscle mass and strength . However, muscle-specific force and muscle activation patterns have been found to be normal in RA patients with stable disease, even with significant muscle loss .
The study has a number of limitations. HHD is widely used and has been shown to be reliable for testing muscle groups in long term chronic conditions in adults and children but not in rheumatic diseases. A major limitation of HHD is the reliance on the tester to hold the device to accommodate the force being generated by the person being tested . Future studies should evaluate both the intra and inter-tester reliability of HHD in rheumatic diseases. Previous RA studies have reported loss of muscle mass, decreased physical activity, and immunological factors may combine with alterations in skeletal muscle properties that could result in decreased muscle strength and functional limitation [18, 19]. Therefore, the results from the current study on muscle strength alone need to be carefully interpreted. The results we found suggest that in a heterogeneous disease such as RA fluctuating symptoms, particularly joint pain and effusions may lead to measurement error. We can postulate that the difference in plantarflexion/dorsiflexion ratios may in fact be as a result of metatarsophalangeal joint synovitis or pain/tenderness which may inhibit the ability to generate force. As our study design was case–control, not longitudinal, it is not possible to know whether progression of the muscle strength decline is linear. Generalisation of these results can only be tentative, as the sample size was small and the patients participating in our study represent individuals with established RA and were all women. In the current study we used the x-ray erosion scores and the positivity of rheumatoid factor and anti-CCP to determine that the RA cohort was well established. A limitation of the study was not using a disease activity measure score such as the DAS28.
Further work should target the relationship between foot and ankle muscle activity and foot and ankle characteristics that include foot pain, foot sensation and intrinsic muscle strength of the foot since a significant number of patients with established RA suffer from foot and ankle problems . Future work should also consider a standardised protocol when using HHD to measure foot and ankle muscle strength in a clinical setting.